Marconiphone VRC74DA

[Heatercathodeshort]

Originally Posted by mark pirate

I must admit this set has been very challenging so far, mainly due to the many faults I have uncovered, and the poor quality of design and components.Mark

All the faults known to man and all on one receiver. It's an 1807 chassis!

If you have the sound on channel 4 check along the vision IF chain, detector and video output stage. The signal generator should sort it out. The video amp can be checked by connecting the audio output from the generator via a .1uf capacitor to the control grid pin 1 of the Z77. Work back at RF from the anode of the vision IF's. John.

[mark pirate]

To establish if the local oscillator is working connect a test meter to the grid of the triode section of the X78, pin7. You can't connect the meter lead direct, this will simply stop the oscillator, so a resistor of say 100K must be inserted in series with the lead. With a working oscillator there should be about 4 volts negative present of the triode grid.

I have just tried this, I only have -2.1v on pin7. The resistor from pin 7 to chassis should be 1 meg, it is reading a bit high at 1.2 meg.
I tried my other X78, the voltage reading on that was even lower!

Mark

[FERNSEH]

Hi Mark,
The X78 triode grid resistor is 0.1megohm (100Kohms) The fact you have negative volts on the pin 7 indicates that the oscillator should be working.
Perhaps the true figure is about -2 volts after all?

DFWB.

[mark pirate]

The X78 triode grid resistor is 0.1megohm (100Kohms)

I must check the range on my meter
I have tried my signal generator via a 0.1 cap through the aerial socket. I can hear the tone as I tune it, but only a barely noticeable effect on the raster.

I will dig out my Marconi TF2015/1 sig genny tomorrow, as it can run on a 24v battery supply. As I am not happy prodding around this chassis with my old Advance E2!

Mark

[Heatercathodeshort]

The E2 should be fine and ideal for this vintage TV. Have you rocked it around the vision IF with the E2 connected to the mixer anode of the X78? It's probably easier to work back from the video output valve as you would the loudspeaker in a radio receiver. John.

[FERNSEH]

Hi Mark,
As John states the E2 will be fine for fault finding in this set. Another trick you could try is first remove the screening can from the X78 and inject the modulated IF signals from the sig gen by winding an open loop around the valve. For starters max output from the sig gen. For the sound rock the sig gen frequency between 37 to 38 Mc/s. Do the same for the vision IF amplifier, sig gen tuned to 34 to 36Mc/s. Connect a volt meter across the video detector load resistor in order to monitor the DC component and to determine if the IF amplifier works. If the video amplifier is OK there will be some modulation apparent on the CRT screen.

DFWB.

[mark pirate]

Hi John, I will be using the E2, as the Marconi genny has decided to pack up rather than be anywhere near this TV

I was a bit worried about the spark when connecting it via a cap to the aerial socket last night, I did not want to trip the electrics and have an accident in the darkness!
I must get my isolation TX back....

After lunch I will lift the earth on the genny, and get injecting some signals as suggested.
I will certainly be happy to find out where the problem lies.

David, I will try winding an open loop around the X78, I guess this would also be a safer way to inject a signal. I will try this first.

I will report back with my findings later.

Mark

[mark pirate]

I am still getting nowhere with this set
I tried the open loop method on the X78, and get the tone loud and clear, but no response on the screen.

I have connected the signal genny via a 0.1 cap to the X77 (pin 5), same result.
The IF Z77's were also injected to pin 1, again no response on screen.
In fact the only sniff of anything on the raster, is when peaking the audio signal on the E2.

any suggestions chaps?

Mark

[FERNSEH]

Hi Mark,
A quick test to ascertain if the video amplifier is working. Momentarily short circuit the cathode resistor of the Z77 video amplifier valve to chassis. This action will cause the valve to pass more anode current and the result if the amplifier is OK, will be more brightness on the CRT.
Also, connect the voltmeter across the video detector diode load resistor. If the IF amplifier working there will be positive volts across the resistor.

DFWB.

[mark pirate]

Hi David, shorting the cathode resistor of the Z77 (V5) does indeed make the raster a bit brighter.
I am getting 79v across the D77 load resistor (from pin 5).

Mark

[Heatercathodeshort]

Hello Mark,
This is becoming far more complicated than need be.
With a vision IF strip/video output stage you need to check to see if the video amplifier is working. To do this it's a simple matter of connecting the E2's audio output via a .1uf capacitor to the control grid pin 1 of the video amp. You should get strong bars on the raster. Remember this signal is fix tuned at audio level generated by a separate oscillator within the E2. It is independant of the carrier oscillator section of the generator.
If OK you can work back along the vision IF amp feeding the vision IF frequency 34mc/s [this time from RF output] into pin 1 of V4 Z77. A reasonably strong signal should be present with slight rocking of the E2. Work back to V3 and check where the signal disappears, anode pin 5 then control grid as V4.
Common faults are: Faulty Z77 valves [EF91] O/C decoupling capacitors from screen grids of Z77 valves pins 7 and burnt feed resistors caused by shorted decouplers in anode circuits. Check the cathode voltages of V3 and V4. Should be around 1.5v if the valves are conducting. [Guessed]
If you are not using an isolation transformer [strongly advised] make sure the chassis is connected to neutral. It sounds as if it is live by the slight sparks at the aerial socket and your E2 connection. John

[mark pirate]

Hi John, I double checked that the set is wired neutral to chassis, which it is.
On checking the extension lead, some muppet (not me) has crossed live and neutral

Back to the set, I have injected pin 1 on the Z77 video output (V5) using the AF output from the E2. It does have an effect on the raster, but not black bars, it just makes it a bit darker.

Going via the RF output on the E2 has an effect, but only around the sound frequency (37.5mhz). At vision frequency (34mhz) there seems to be no effect.
This is the same for the IF valves (pin 1).

Cathode voltage is 3v and 5v on the IF Z77's.

Mark

[Heatercathodeshort]

What is the effect of feeding 34mc/s [modulated] to the grid of V4 pin 1?

[mark pirate]

It just darkens the raster a bit, sweeping around the frequency makes little difference.

I am working from the circuit that david posted, but I really need to tie up the circuit with the physical layout regarding valve layout.
The layout below is for the london model, which is different to mine.

Mark

[FERNSEH]

Originally Posted by mark pirate

Cathode voltage is 3v and 5v on the IF Z77's.

That seems a bit high, but saying that the cathode bias resistors are 680 ohms.
The usual value is 180 ohms.

With the signal generator disconnected and no aerial input and with the volt meter connected across the vision detector load resistor R24, What is the voltage reading? If there any volts across R24 then it is likely the IF amplifier is unstable and if that's the case check the decoupling capacitors associated with V3 and V4.

DFWB.

[mark pirate]

With the signal generator disconnected and no aerial input and with the volt meter connected across the vision detector load resistor R24, What is the voltage reading?

0.03v steady.

Mark

[FERNSEH]

0.03V, that's OK. Time to start injecting signals into the final IF amplifier. Keep the meter connected across R24. Force feed V4 with 36Mc/s, that is max output from the sig gen into pin 1 of the Z77. Note the reading on the meter. Rock the sig gen between 33 to 37Mc/s.

DFWB.

[mark pirate]

I have just got back to this set, but still no joy getting any real response on the screen no matter what I have tried. This by set seems to have me beaten
I have tried injecting both AF and RF from the signal generator to all Z77 valves on pin 1 (G1) and pin 5 (Anode) with no more than a mild effect on the raster, even with the genny on full output.


I really need to go step by step with this problem. Using the circuits posted by David, I have drawn up a valve layout to match so we can be sure where we are with valve numbers.

If it comes to it, I can borrow the RF deck from my HMV set, I know that does have sound, but the vision side is unknown. It has not been touched, so still has the original caps.

Mark

[Freya]

But you must be getting something on V6 pin 1, if not what are the voltages on that valve ?

[mark pirate]

Hi Stephen, I will let you know. I intend to get back to the set this afternoon.
I need to thoroughly go through the chassis with my meter, there must be an open circuit somewhere, as there is so little response to such a large signal.

At least the audio is getting through fine!
I have taken several detailed photos of the unrestored RF deck in my HMV set, this is an identical Midlands unit,apart from an additional (separate) preamp stage.
So at least I have something to compare my faulty unit with.

Mark